An above-mentioned apparatus is described in EP 1 770 219 A1. In addition to the first guide elements, this apparatus has second guide elements which abut against the side of the wall of the second borehole that faces the first borehole.
The guide elements accordingly interacting with two boreholes are intended to prevent the boreholes, which partially intersect each other in cross section, from diverging from their mutually parallel direction to the extent that the opening between the boreholes that is generated by the intersection of the cross section is greatly reduced or even disappears altogether. This result of departing from the parallel orientation of adjacent boreholes relative to one another is very particularly disadvantageous in particular when the adjacent boreholes belong to a large number of correspondingly made boreholes which, once made, are to be filled out with suitable sealing material, for example concrete, in order to produce an impermeable sealing wall. The reason for this is that if the connecting opening were reduced, the sealing wall would be weakened in an undesired manner; in the case of complete disappearance of the connecting opening, the sealing wall would even be interrupted. The structure, which is often complex to produce, would then not perform its purpose.